I've also bought a CIB Berzerk in not so good shape (very scratchy overlay) from ebay, a MATEOS flash cart system, and a 72in1 multicart (I've in the process of trading it for another multicart with madtronix, because the my cart does not work on my no-buzz Vectrex). I've ordered Vector Pilot with one overlay, YASI\Protector and Nebula Commander, but they have not arrived yet.
Well damn, that'll keep you (and the kids) quite busy for some time!
At present, a new, never used MineStorm overlay might not be worth much more than a used one but there is a possibility that it might be worth something in the future if collecting for the Vectrex really takes off. Nice find though.
Take a picture of it sealed. Most of the Sealed Vectrex units probably stay sealed, so sealed overlays are seen often.
I once inherited a set of NOS taillights for a 69-73 Opel GT. Pretty rare to come across a set NOS. They're reproduced with very high quality, so it was not necessary (to me and my cars) to keep them. I sold them, made someone who cared about the NOS ones happy and bought the repros for my own use. I think that they went to the place that they belonged.
You may not consider yourself to be a collector, but the Vectrex is one hell of a collector's piece and a rarity among retro gamers. In other words, you're part of an esteemed club now, so I hope you enjoy the community and the awesome games that are coming your way.